A gate valve has a body with a central cavity that is intersected by two coaxial passages. Seat rings are located at the intersections of each passage with the cavity. Each seat ring has a face that is engaged by a gate that moves between an open and a closed position. In some cases, these gate valves are subject to high bearing loads due to high pressure forcing the gate against the seat ring. Moving the gate under high loads may lead to galling or marring the surface finish of the gate-seat interface. Galling increases the friction between the gate and seat ring and reduces the sealability. Premature failure may result.
A ball valve also utilizes a seat ring with a face that slidingly engages a sealing element, which in this case is a ball element. The face of the seat ring tends to gall at its inner edge.
All materials in normal bearing contact will experience failure or damage to the surface finishes if loaded to a high enough level when subject to dynamic motion. A ball or gate valve must be able to cycle while maintaining a certain leak tightness. It is therefore advantageous to have valve components that experience dynamic contact during operation that are resistant to surface damage.
It is also well established that lubricants, such as grease, have diminished capacity to provide lubrication as the bearing stress becomes extreme. This is because the surface tension and viscosity of the fluid are unable to support the high loads resulting in surface contact and subsequent surface damage. This effect is amplified at high valve operating temperatures, where the lubricant's viscosity and surface tension are reduced.